Engine exhaust emissions are becoming increasingly important for engine manufacturers. Governments and regulatory agencies are enforcing ever more stringent emissions standards for many types of highway and off-highway vehicles, and manufacturers must develop new technologies to meet these standards while providing high-performance, cost-effective equipment to consumers.
One significant exhaust gas pollutant includes nitric oxides (NOx). Engine manufacturers use a variety of different technologies to decrease NOx emissions including new engine designs that produce low levels of NOx during combustion and exhaust system technologies that remove NOx from exhaust gases before release to the environment. One technology for removing NOx from engine exhaust is selective catalytic reduction (SCR). SCR works by causing NOx to react with a reductant (e.g. ammonia) to produce environmentally friendly products, such as nitrogen gas and water.
SCR presents its own challenges. For example, SCR requires a reductant, which must be carried in an on-board tank or produced during machine operation. However, supply of a reductant through an on-board source requires periodic replenishment, which can be inconvenient and expensive. In addition on-board production may be complicated by difficulties in appropriately matching reductant and NOx levels or by an inability to produce enough reductant. Regardless of how the reductant is provided, it is desirable to control the amount of NOx produced by the engine to appropriately match reductant and NOx levels at a downstream SCR catalyst.
One NOx emission control system is described in U.S. Pat. No. 6,882,929, which issued to Liang et al. on Apr. 19, 2005 (hereinafter the '929 patent). The method of the '929 patent includes determining predicted NOx levels based on a model reflecting a relationship between NOx levels and a number of control parameters. The method may further include reducing engine NOx production based on the model.
Although the method of the '929 patent may allow reduction in engine NOx production, the method of the '929 patent may not provide suitable control of NOx levels for SCR. For example, while the method of the '929 patent may provide a feedback mechanism for reducing NOx production, it may not allow rapid adjustment of NOx within a desirable range so that NOx and reductant levels may be matched appropriately at a downstream catalyst.
The present disclosure is directed at overcoming the shortcomings of the prior art NOx emissions-control systems.